Hi
I'd recommend you to check each and every notification in settings-->notifications center. Sounds like you may have an app notification to not show in lock screen or notification center or show banners/alerts, just to make an alert sound/vibrate. I had the same situation not long ago and it turned out to be just that.

I would agree with this statement as there are many apps I will get a very quick short burst (Vibrate) to let you know something mostly like news or sports and some times there are no references built into the notification but just something the DEV put in.

OP, I have been experiencing the same thing. I'll get a notification sound, but can't figure out what the notification is for. I generally just let it go. Especially since my phone is generally on silent lol.